External-cavity semiconductor lasers, including lasers with frequency selective tuning elements in the cavity, are well known and have been extensively studied. For example, T. Fujita, et al., in Applied Physics Letters 51(6), pages 392-394 (1987), describe a laser having a buried heterostructure laser that has been antireflection (AR) coated on the intracavity facet, a collimating lens, a polarization beamsplitter, external cavity mirrors in each of the TE and TM polarization light paths, and an electro-optic modulator in the TE polarization path between the beamsplitter and cavity mirror. The configuration allows selection of either the TE or TM mode of oscillation by adjusting the modulator's bias voltage. W. Sorin, et al., in Optics Letters 13(9), pages 731-733 (1988), describe a laser having a laser diode with one of its facets AR coated to reduce its reflectivity, a lens, a single mode optical fiber and a tunable evanescent grating reflector for providing feedback. The laser is wavelength tunable by sliding the feedback grating laterally over the fiber. P. Zorbedian et al., in Optics Letters 13(10), pages 826-828 (1988), describe another wavelength tunable laser using either a rotatable interference filter in an external Fabry-Perot cavity or an external grating reflector providing frequency-selective feedback.
A problem with previously available external-cavity semiconductor lasers is their generally low output power (on the order of 10 mW cw and 200-300 mW pulsed). Further, higher output powers are associated with unstable output intensity and frequency and less than good modal quality.
In U.S. Pat. No. 4,251,780, Scifres et al. describe semiconductor injection lasers that are provided with a stripe offset geometry in order to enhance and stabilize operation in the lowest order or fundamental transverse mode. In one configuration, the stripe geometry has a horn shaped or trapezoidal section connected to a straight section, in which the width of the horn shaped or trapezoidal section expands from 8 .mu.m at the straight section to 25 .mu.m at the cleaved end facet. In contrast to configurations in which the edges of the stripe waveguides are linear and orthogonal to the cleaved end facets of the lasers, the nonorthogonal angled or curved edges of the offset stripe geometries cause higher order modes to reflect or radiate out of the waveguide, thereby increasing the threshold of the higher order modes relative to the fundamental mode.
In U.S. Pat. No. 4,815,084, Scifres et al. describe semiconductor lasers and laser arrays in which lenses and other optical elements have been integrated into the semiconductor bodies of the lasers by means of refractive index changes at boundaries in the light guiding region, where the boundaries are characterized by a lateral geometric contour corresponding to surfaces of selected optical elements so as to cause changes in shape of phase fronts of lightwaves propagating across the boundaries in a manner analogous to the change produced by the optical elements. In one embodiment, a biconcave or plano-concave diverging lens element is integrated within the laser in order to counteract the self-focusing that usually occurs in broad area lasers and that can lead to optical filamentation and lateral incoherence across the laser. The diverging lens in the laser allows the laser to operate as an unstable resonator, leading to high output power and good coherence across the lateral wavefront.
An object of the invention is to provide a high power, external cavity, semiconductor laser which emits a single spatial mode, diffraction-limited output beam.
Another object of the invention is to provide a wavelength tunable, high power, external cavity, semiconductor laser with a stable, single frequency, narrow linewidth light output.